The invention relates to a magnetic resonance imaging apparatus for generating and detecting magnetic resonance signals, comprising a magnet for generating a steady magnetic field, a gradient coil system for generating gradient fields, and means for correcting image artefacts caused by local field aberrations.
A magnetic re imaging apparatus of this kind is known from U.S. Pat. No. 4,591,789. In an apparatus disclosed therein, field aberrations are measured and on the basis thereof a location determined by the local field, to be referred to hereinafter as field location, of an object point is shifted to a geometrically appropriate location. Correction can thus be made for strictly geometrical, monotonous aberrations of gradient fields within an examination space. For inhomogeneities of field gradients, for example homogeneities such that unintentially the same field strength occurs in several geometrical locations, correction is not possible in this way. This is because several regions of an object are then imaged in a single image region due to equal field strengths at points in different regions. Image defects, also referred to as artefacts, also occur, for example because resonance signals from sub-regions of an object which are situated outside a volume to be examined unduly contribute to the imaging due to their same field location as object regions situated within the volume to be examined. The latter phenomenon will occur notably in apparatus comprising a magnet system having a comparatively small transverse dimension with respect to an object to be examined, so that the distance between peripheral object regions and current windings of the coil system are comparatively small. Similar errors may occur due to field strength aberrations in the main magnetic field near axial ends of the magnet. A gradient field applied at that area can, for example be locally compensated, so that the position determination is lost. These artefacts will occur notably in magnet systems having a comparatively small axial dimension.
The occurrence of a part of the artefacts can be prevented by providing a magnetic resonance imaging apparatus with a magnet system having a large transverse dimension and a long length with respect to objects to be measured, for example patients to be examined. Notably a larger diameter is extremely unattractive because of the power required; when a superconducting main magnet is used, notably the power for applying gradient fields is important. A longer magnet makes the apparatus expensive and patient-unfriendly.